Processing, Storage and transportation of Cryogenic fluids such as LNG require the use of materials that have (1) adequate low temperature fracture toughness to ensure against the risk of brittle fracture process, contain, and transport fluids at cryogenic temperatures and (2) adequate strength to hold the fluid pressures without the need for excessive wall thickness. In order to maintain the fluids at the cryogenic temperature during storage and transportation, insulated vessels and pipelines are requires.
Pipe-in-pipe (PIP) assemblies with insulation between the inner and outer pipes are used for the transportation of cryogenic fluids. The inner pipe can be subject to cryogenic temperatures which is −165° C. for LNG while the outer pipe is subject to the atmospheric temperature which can vary between 4° C. to +30° C. depending on whether the PIP is installed underwater or on-land. Because the large temperature differential between the inner and outer pipes of the PIP system the differential thermal contraction between the inner and the outer pipes is considered as critical design factor. The current practice to manage the differential contraction is to install contraction spools or external bellows if the PIP in installed above ground and internal bellows if the PIP is buried. A typical liquefied natural gas (LNG) pipeline utilizes pipe-in-pipe assemblies consisting of 304 stainless steel inner pipe, carbon steel outer pipe and with polyurethane foam insulation. Because the complexity of the contraction spools and bellows, the industry has been considering the use of the specialty 36% nickel alloy, also known with the trade name INVAR, instead of the commonly used 304 SS because the coefficient of thermal expansion of 36% Ni alloy is about one tenth of the 304SS. The use of 36% Ni alloy simplified the PIP design particularly for subsea PIP because it eliminated the need for contraction spools and bellows.
The coefficient of thermal expansion of 304 SS is 17.2×10−6/° C. and when a pipe is cooled to the LNG temperature of −165° C. from room temperature of 20° C., it will contract by 2.5 m per km of pipe length. If one is to hold the pipe at its ends to prevent it from contracting, one needs to apply a stress on the pipe in the order 75,000 pounds per square inch of the pipe cross sectional area which, assuming the pipe can be supported to prevent its buckling, is very high. To accommodate this contraction without imposing this high stress or causing buckling, contraction loops or bellows are used. The other option is to use the expensive 36% Ni alloy that has low coefficient of thermal expansion of less than 0.9×10−6/° C. and thus controlling its contraction will only require imposing a stress that is less than about 6,000 pounds per square inch of the pipe cross sectional area.
When higher reliability is required, particularly for LNG pipelines that are installed offshore or near residential areas, double barriers are considered by using two inner pipes with insulation between them. This construction is known as a pipe-in-pipe-in-pipe (PIPIP) configuration. The first inner pipe is the primary barrier but in case it leaks the secondary containment is provided by the second inner pipe. Insulation is provided between the second inner pipe and the carbon steel outer pipe. For this construction the use of bellows and contraction spools becomes too complicated. Carbon steel pipe secondary containment is the practice whereby the second pipe is used to provide an additional level of containment should the inner pipe fail or leak. For this to be possible, the second pipe (either intermediate or outer pipe) cannot be carbon steel, as it would fail due to the thermal shock loads. The secondary containing pipe is therefore required to be made of more ductile stainless steel so as to withstand an individual accidental loading down to the minimum LNG operating temperature (−165° C.).
Insulation between the inner cryogenic pipe and the outer steel pipe in case of PIP and between the two inner cryogenic pipes and also between the second cryogenic pipe and the external steel pipe in PIPIP is provided by mechanical insulation such as polyurethane foam or aerogel type materials or by vacuum or by combination of both mechanical insulation and partial vacuum.
A need exists for an alternative to allow the use of low cost cryogenic materials such as 304 SS without the need for the complexity of the contraction loops or the bellows and control the differential contraction of the stainless steel and without the need to resort to the use of the expensive 36% nickel material. The primary object of the present invention is to provide a cryogenic pipeline that is made of low cost materials such as 304 SS but performs as 36% nickel when cooled to the cryogenic temperature.